DESCRIPTION: (Adapted from the application) Actions of estrogen on function of endothelial NOS may account for cerebrovascular protection. Thus, the first hypothesis is: 1) cerebrovascular responses involving NOS are affected by estrogen. Rat middle cerebral arteries in vitro will be used to test the influence of sex and gonadal steroids, comparing males and cycling females, control or gonadectomized and steroid-replaced. The investigator will compare myogenic tone in arteries from males and females and test whether NOS inhibition abolishes gender or estrous cycle differences. Possible K+ channel involvement will be investigated. Since shear stress stimulates the activity of NOS, the PI will test whether flow induced vasodilation is influenced by estrogen status and investigate modulation by gender and gonadal steroids. As a control, effects of gender and gonadal steroids on vasodilator responses to increased extracellular K+, which does not involve NOS will be investigated. The second hypothesis is: 2) estrogen acts by regulating levels of eNOS protein. Responses to endothelial dependent vasodilators, levels of NOS protein by Western blot and NOS activity will be measured, and NOSIII-deficient mice will be studied. For the third specific aim, the PI will test whether: 3) estrogen modulates effects of melatonin by a selective action on MT2 receptor mediated vasodilation. Circadian variation in cardiovascular disease onset has been shown and estrogen regulates vascular sensitivity to melatonin. The PI will determine whether melatonin causes constriction of cerebral arteries through MT1 and dilation via MT2 receptors. Influence of sex, transmural pressure, artery size and endothelial factors will be investigated. The PI will also test whether dilator responses via MT2 receptors are preferentially enhanced by estrogen exposure and using RT-PCR whether gonadal hormones alter expression of melatonin receptor subtypes. The final hypothesis is: 4) effects of estrogen are mediated by the classical estrogen receptor Estrogen receptor antagonists and estrogen receptor knockout mice will be used to test whether this receptor accounts for effects on cerebrovascular reactivity. Given the enormous impact of hormone replacement therapy, knowledge of the effects of gonadal steroids and melatonin on the cerebral circulation is essential.